1. Field of the Invention
This invention relates to a headlight assembly for a vehicle, and in particular to a headlight assembly for a vehicle comprising a pair of headlight units each having first and second headlights and a lighting control circuit for these headlights.
2. Description of the Prior Art
A conventional headlight uses a halogen bulb or a discharge bulb as a light source thereof. A headlight using a halogen bulb has an advantage that response speed is superior since a target amount of light can be obtained in a short time after applying a voltage to a filament thereof. On the other hand, however, there is the disadvantage that a large amount of electrical power is consumed when used.
A headlight using a discharge bulb has an advantage that the amount of light is large relative to the amount of electrical power consumption. As a result, it is possible to compact the size of the headlight by reducing the size of a reflector of the headlight. Further, there is also an advantage that the degree of durability is superior in comparison with a halogen lamp. On the other hand, however, there is the disadvantage that repeated switching on and off causes a shortened life. Further, there are also disadvantages that this type of headlight is inferior in responsiveness since arc discharge is required at the start of lighting, and that the lowering of supply voltage would occasionally cause turning-off or unlighting of the bulb.
In view of the advantages of these two types of bulbs, an improved headlight assembly which utilizes these two types of bulbs has been developed, which is for example disclosed in Japanese Patent Laid-Open Publication No. 134020/1984.
The developed headlight assembly comprises a pair of headlight units provided on left and right sides of a front portion of a vehicle, respectively, and a lighting control device for the headlight units. Each of the headlight units includes a first headlight having a single-filament halogen bulb and a reflector and a second headlight having a discharge bulb and a reflector, thus this headlight assembly is called a four headlights type. In this conventional four headlights type headlight assembly, the discharge bulbs are turned on during dimmer lighting which illuminates a range in the vicinity of the vehicle with low-beam. During main lighting which illuminates a range faraway from the vehicle with a high-beam, the halogen bulbs are turned on.
In the meantime, recently compactification of the size of the headlight assembly, especially the vertical length of the headlight unit, is requested in view of improvement of the design and appearance of a vehicle. In such a case, this conventional headlight assembly does not satisfy the request. Namely, in order to reduce the vertical length of the headlight unit, it is also necessary to reduce the vertical length of a reflector of each headlight. A reflector having a reduced vertical length may be suitable for the discharge bulb due to one of the advantages of the discharge bulb, i.e. its large amount of light with respect to the power consumption. However, this reduced size reflector is insufficient for the halogen bulb to produce a sufficient amount of light during the main lighting with the electrical power suitable to the discharge bulb. Namely, as stated hereinabove, the halogen bulb requires a relatively large amount of electrical power to be consumed. Therefore, it is difficult for the halogen bulb to produce a sufficient amount of light during the main lighting with the reduced size reflector and the electrical power suitable to the discharge bulb. For this reason, it has been difficult to shorten or reduce the vertical length of the headlight unit in the conventional headlight assembly.
In order to reduce the vertical length of each headlight unit, it may be proposed that both the discharge bulbs and the halogen bulbs be illuminated during the main lighting. However, in such a proposed headlight assembly, there is a problem that if passing is effected by repeatedly flashing the headlights, for instance, as a warning signal when overtaking a vehicle running in front of the vehicle, the discharge bulbs also have to be repeatedly switched on and off, thus resulting in shortening the discharge bulbs' life for the reason stated above.